Pyrotechnic gas generator compounds

ABSTRACT

The main subject of the present invention is a solid pyrotechnic gas generator compound, the composition of which contains, expressed in weight percentages:
         from 60 to 70% of guanidine nitrate,   from 26 to 33%, advantageously 26 to 30%, of potassium perchlorate,   from 2.5 to 6% of at least one combustion modifier chosen from transition metal oxides, the precursors of such oxides and mixtures thereof,   from 0 to 6% of at least one additive, and does not contain an explosive ingredient.       

     Such a compound is particularly effective for the inflation of side airbags.

A subject of the present invention is pyrotechnic gas generator compounds, suitable for use in motor vehicle occupant protection systems, more especially for the inflation of airbags and most particularly for the inflation of side airbags (see below).

The technical field relating to the motor vehicle occupant protection has experienced a very large expansion over the last twenty years. The latest generation of vehicles from now on integrate within the passenger compartment several safety systems, of airbag type, the operation of which is carried out by the combustion gases of pyrotechnic compounds. Among the airbag-type systems, front airbags for front impacts and side airbags for side impacts are mainly distinguished.

The side airbag systems differ from the front airbag systems essentially due to the time required for the deployment and positioning of the airbag. Typically, this time is shorter for a side airbag (about 10-20 ms, compared with 40-50 ms for a front airbag). For a side airbag, the functional requirement of inflation of the bag over a short time makes it necessary to resort to a pyrotechnic composition having a sufficiently high combustion rate (typically equal to or greater than 30 mm/s, or even 35 mm/s, at 20 MPa) over the operating pressure range in the combustion chamber of the generator, in order to obtain a sufficient value of the inflation rate per unit area (product ρ×n×Tc×Vc). Moreover, in order to ensure a satisfactory start-up of the system, the pyrotechnic composition must also have good ignitability characteristics. Also, given the generally tapered surface profile of the charges used (of pellet type), the composition should ideally have a combustion rate that is stable and high enough at low pressure. In fact, those skilled in the art are aware that the specifications for side airbags are more restrictive than those for front airbags. Of course, any technological advance in the field of said side airbags can also advantageously be available in the field of front airbags.

In the present text, the term “low pressure” is used to define a pressure P such that: 0.1≦P<10 MPa, the term “medium pressure” is used to define a pressure P such that: 10 MPa≦P<30 MPa, and the term “high pressure” is used to define a pressure P such that: P≧30 MPa.

It is, moreover, customary to compensate for the low combustion rates of the pyrotechnic compounds used in the current gas generators for airbags by having recourse to charges composed of pellets with very small dimensions. Although not cost effective owing to the low production throughput by weight of pelleting machines and the tooling costs generated, this makes it possible, to a certain extent, to partly overcome the lack of combustion rate. However, this solution accentuates two other drawbacks:

-   -   difficult ignition, which is increased owing to the high initial         surface area of the charge, which then requires the use of a         reinforced igniter or the addition of an additional charge         acting as an ignition relay;     -   a strong tapering of the combustion surface owing to the small         size of the pellets which creates a long combustion tail at low         pressure. This long combustion tail at low pressure is the         source of the emission of the majority of the toxic species         present in the gases used to inflate the bag.

It should therefore be noted that the desired increase in combustion rate of the pyrotechnic compound in question, over the entire pressure range, including at low pressure, is therefore necessary not only to increase the gas flow rate in order to achieve the inflation delay specifications, but also to ensure the ignitability of the compound without recourse to the use of a relay charge and the innocuousness of the combustion products.

Furthermore, constraints exist with reference to the combustion temperature.

In general, said combustion temperature must not be too high (it must at a minimum remain less than 2400 K, more preferably less than 2350 K) so that the temperature of the gases in the airbag does not harm the physical integrity of the occupant. A low combustion temperature makes it possible, on the one hand, to limit the thickness of the bag and, on the other hand, to simplify the design of the gas generator by making it possible to reduce the presence of baffles and of filters within said generator.

The side airbag systems may call for two types of gas generators: those which are said to be entirely pyrotechnic (the gas generation then being exclusively provided by the combustion of a pyrotechnic charge) and those said to be “hybrid” (the gases then originating jointly from the combustion of a pyrotechnic charge and from a volume of neutral gas stored under pressure in a leaktight reservoir). For “hybrid” generators, the pyrotechnic charge must not have a too low combustion temperature so that the combustion gases are hot enough to compensate for the drop in temperature generated by the volume expansion of the precompressed neutral gas. Ideally, combustion temperatures above 2000 K are required.

Thus, those skilled in the art are in search of pyrotechnic compounds which are suitable for use in entirely pyrotechnic gas generators or in hybrid generators, more particularly intended for side airbags, i.e. which simultaneously have a moderate combustion temperature of about 2000-2400 K, more preferably 2000-2350 K, and a high combustion rate over the entire operating pressure range (in particular greater than 30 mm/s at 20 MPa, more preferably greater than 35 mm/s at 20 MPa), including at low pressure.

In addition, the pyrotechnic compounds for airbags must also aim to jointly meet the following requirements:

-   -   the gases generated by the combustion of the pyrotechnic charge         (comprising a compound or n compounds) must be nontoxic, i.e.         have a low content of carbon monoxide, of nitrogen oxides and of         chlorinated compounds;     -   the gas yield (i.e. the amount of gas generated by the         combustion) must be high in order to result in a high inflation         power;     -   the amount of solid particles generated by the combustion,         capable of constituting hot spots that may damage the wall of         the airbag, must remain low;     -   the pressure exponent must be as low as possible, in particular         at medium and high pressure (typically below 0.35 as described         in the prior art) but also at low pressure. A low pressure         exponent in fact makes it possible to very significantly reduce         the operating variability between the low temperature extreme         (around −40° C.) and the high temperature extreme (around 90°         C.) that are required in the field of use. The operating         reproducibility is as a result improved and the size of the         metal structure of the generator can advantageously be reduced;     -   their cut-off combustion pressure (their combustion limit         pressure) must also be as close as possible to atmospheric         pressure.

It is also highly desirable for the basic ingredients of the compounds not to be dangerous from the pyrotechnic point of view. The presence of ingredient(s) belonging to the class of explosives, such as nitroguanidine, hexogen (RDX) or octogen (HMX), is advantageously avoided. The term “explosive ingredients” means ingredients classified in risk division 1.1 according to standard NF T 70-502 (see also UNO—Recommendations relating to the transport of dangerous goods—manual of tests and criteria, fourth revised edition, ST/SG/AC.10/11/Rev.4, ISBN 92-1-239083-8155N 1014-7179 and STANAG 4488). Guanidine nitrate and potassium perchlorate, taken separately, are not ingredients classified in this risk division. They do not constitute explosive ingredients, in particular within the meaning of the invention.

It should at this stage be pointed out that the incorporation of highly energetic explosive ingredients, such as nitroguanidine, even at low levels, prejudicially contributes to increasing the combustion temperature of the compounds, beyond the cut-off threshold of 2350 K set by the need for technical improvement desired by the inventors. Thus, compounds as described in U.S. Pat. No. 6,893,517, consisting mainly of a mixture of a guanidine derivative (preferentially guanidine nitrate), of an explosive nitrogenous compound (preferentially nitroguanidine) and of an inorganic oxidizing agent (such as ammonium perchlorate or potassium perchlorate), do not meet the requirements of the specifications of the present invention. These compounds also include in their composition a low level of a ballistic catalyst, consisting of an oxygen-containing compound of a transition metal, advantageously with a high specific surface area, conventionally used in the field of propellants for increasing the combustion rate at medium and high pressure (this catalyst accelerates the decomposition of the oxidizing charge). The gas microgenerators for seatbelt tensioner devices as described in said U.S. Pat. No. 6,893,517 (and in its priority application EP 1 275 629) operate via pulses, which requires a high combustion rate at medium and high pressure. A high pressure exponent at low pressure and noncombustion at atmospheric pressure of the compounds in question does not pose a problem, since the pressure does not fall again, in the context of the use of said compounds, to a low level before the end of the pyrotechnic operation. This application for seatbelt tensioner devices does not need, for the gas generator, requirements as severe as those needed in the context of the present invention (airbags, and most particularly side airbags), most particularly a high combustion rate at low pressure, a drop in the cut-off combustion pressure threshold (as close as possible to atmospheric pressure) and a low pressure exponent over the entire combustion range (in particular at low pressure).

Currently, for front airbags, the pyrotechnic compounds which offer a good compromise, in terms of gas temperature, gas yield, level of particles emitted and toxicity, contain, as main ingredients, guanidine nitrate (GN) as reducing charge and basic copper nitrate (BCN) as oxidizing charge. U.S. Pat. No. 5,608,183 and U.S. Pat. No. 6,143,102 describe such compounds.

However, these compounds have relatively low combustion rates, less than or equal to 20 mm/s at 20 MPa, and also a low gas yield. They are also difficult to ignite.

From the perspective of improving the ignitability of compounds of this type, the addition of perchlorate to their composition based on guanidine nitrate (GN) and on basic copper nitrate (BCN) has been proposed according to the prior art. Thus, patent application EP 1 526 121 describes the addition of a perchlorate (in particular potassium perchlorate), in a low amount (less than 5% by weight), for improving the ignition of these compounds. However, the incorporation of perchlorate in such a low amount does not make it possible to sufficiently increase the combustion rate of the compound for satisfactory use in gas generators for side airbags.

Applications WO 2007/042735 and WO 2009/126702 describe compounds having compositions, of the same type, which contain guanidine nitrate (GN), as reducing charge, basic copper nitrate (BCN), as main oxidizing charge, and, in addition, a second oxidizing charge, which advantageously consists of potassium perchlorate (KClO₄). These documents associate the good performances of said compounds, in particular a high combustion rate at high pressure, with the composition but also with the specific process for producing said compounds (which process includes a dry roller compacting step for WO 2007/042735 and two successive spray-drying and compression steps for WO 2009/126702).

From the perspective of improving the gas yield and the combustion rate, compounds based on one (or more) nitrogenous reducing agent(s) combined with a strong oxidizing agent of perchlorate type have also been proposed according to the prior art.

Patent application US 2006/0137785 describes the combination of a reducing agent of guanidine type (nitroguanidine or guanidine nitrate) and of ammonium perchlorate, the latter being necessarily incorporated in a significantly high amount (30% to 60% by weight). The incorporation of ammonium perchlorate in such a high amount results in two major drawbacks which are, firstly, a significant increase in the combustion temperature (above 2800 K) and, secondly, the generation of hydrogen chloride (which is a toxic and highly corrosive gas), said hydrogen chloride then being present in the gas effluents. In order to overcome this problem, said patent describes the need to add to the mixture of guanidine+ammonium perchlorate type a metal compound of iron oxide type in order to neutralize the hydrochloric acid present in the combustion gases, which results in a decrease in the gas yield value for the compound.

The incorporation of potassium perchlorate in place of ammonium perchlorate would have the advantage of resulting in the formation of potassium chloride (KCl) in place of hydrogen chloride (HCl) (but the drawback of reducing the gas yield). In any event, the incorporation of KClO₄ in such high amounts (up to 60% by weight) would produce an increase in the combustion temperature that is totally unacceptable in the context of the intended application.

Logically, those skilled in the art have therefore turned to compounds consisting of a mixture containing guanidine nitrate (GN, alone or combined with a coreducing agent) and potassium perchlorate (KClO₄) in intermediate amounts of approximately 25% to 45% by weight, as described in patent application WO 95/25709 and U.S. Pat. No. 5,854,442 and U.S. Pat. No. 5,997,666, said mixture making it possible to obtain compounds which partially meet the essential requirements of the field of application targeted by the present invention, namely:

-   -   a good gas yield;     -   a moderate combustion temperature;     -   an intrinsic nontoxicity of the particulate effluents; and     -   a combustion rate at around 20 MPa which is slightly increased         compared with the compositions formulated on the basis of         guanidine nitrate (GN) and basic copper nitrate (BCN), but which         remains insufficient for use in side airbags.

The thermodynamic and ballistic characteristics of such a compound (reference compound 1), the (“binary”) composition of which contains only guanidine nitrate (GN) and potassium perchlorate (KClO₄), are given in table 1 hereinafter.

TABLE 1 Ingredients Guanidine nitrate % by weight 68.0 Potassium perchlorate % by weight 32.0 Characteristics Oxygen balance % −3 Density g/cm³ 1.67 Combustion T at 20 MPa K 2351 Gas yield at 1 bar - 1000 K mol/kg 33.2 Combustion rate at 8 MPa (low pressure) mm/s 20.6 Combustion rate at 20 MPa (medium pressure) mm/s 26.3 Combustion rate at 50 MPa (high pressure) mm/s 34.9 Pressure exponent determined between 6 and 0.26 52 MPa Gas content at 1 bar - 1000 K % 82.5 KCl content % 17.1 Cut-off combustion pressure (1) MPa 1.7 (1): the value given is a relative pressure. A zero cut-off combustion pressure corresponds to atmospheric pressure.

Reference compound 1 exhibits many advantages among those expected of a compound for generating gas for an airbag system. The basic ingredients are simple and readily available, inexpensive, safe with regard to the pyrotechnic aspects (no constituent belonging to the explosive compounds class) and nontoxic. The thermodynamic performance (gas yield, particle content) is good and the combustion temperature remains moderate and therefore acceptable. The particles emitted by the combustion are nontoxic (essentially KCl).

However, such a compound does not exhibit the entire performance expected, in particular for a side airbag application. First of all, the combustion rate of about 26 mm/s at 20 MPa is increased only by 20 to 30% compared with that of a compound based on guanidine nitrate (GN) and on basic copper nitrate (BCN), and remains low with respect to the set specifications. Next, during tests with this reference formulation, it emerged that, while the total replacement of the basic copper nitrate (BCN) oxidizing agent with potassium perchlorate (KClO₄) makes it possible to increase the combustion rate above 5 MPa and therefore to improve the ignitability, it in return and highly prejudicially induces a very high pressure exponent at low pressure (greater than 0.55 over the range 6 to 10 MPa) and noncombustion at atmospheric pressure (additional tests showed that the cut-off operating pressure is around 1.7 MPa, whereas a compound formulated on the basis of guanidine nitrate (GN) and of basic copper nitrate (BCN) advantageously has a nonzero combustion at atmospheric pressure).

Starting from the known performance of the guanidine nitrate (GN)/potassium perchlorate (KClO₄) mixture, the inventors wished to propose improved pyrotechnic gas generator compounds which are suitable most particularly for use in side airbags. They more particularly set themselves the objective, while retaining or improving the other characteristics, of significantly improving the following three points:

-   -   decrease in the cut-off combustion pressure,     -   decrease in the pressure exponent (<0.26), advantageously a         large decrease (≦0.2), very advantageously a very large decrease         (≦0.1) from 6 MPa,     -   increase in the combustion rate over the entire pressure range,         in particular at low pressure.

Completely unexpectedly, it proved to be the case that the presence, in the composition of the compounds of the invention, of a low content of (at least) one oxygen-containing compound of a transition metal (a transition metal oxide or a compound which is a precursor of such an oxide), advantageously with a high specific surface area (conventionally used as a ballistic catalyst in the field of propellants for increasing the combustion rate at high pressure (by accelerating the decomposition of the oxidizing charge)), (also) has major effects on the desired three points of improvement above (namely, an increase in the combustion rate (also) at low pressure, a decrease in the cut-off combustion pressure and a decrease in the pressure exponent over the entire pressure range).

The compositions of the pyrotechnic gas generator compounds of the invention (which are most particularly suitable for airbag, in particular side airbag, applications) contain:

-   -   guanidine nitrate, and     -   potassium perchlorate.

They are characterized in that they contain, expressed as weight percentages, from:

-   -   60 to 70% of guanidine nitrate,     -   26 to 33%, advantageously 26 to 30%, of potassium perchlorate,     -   2.5 to 6% of at least one combustion modifier chosen from         transition metal oxides, the precursors of such oxides and         mixtures thereof,     -   0 to 6% of at least one additive, and         do not contain an explosive ingredient.

According to one variant, compositions of compounds of the invention consist (exclusively) of the ingredients listed above (GN+KClO₄+at least one combustion modifier+optionally at least one additive), taken in the contents indicated above.

The ingredients of the first three types above (guanidine nitrate, potassium perchlorate and specific combustion modifier) generally represent more than 90% by weight of the total weight (of the composition) of the compounds of the invention, very generally at least 94% by weight, or even more than 98% by weight. The optional presence of additive(s), such as manufacturing aids (calcium stearate, silica, for example), is expressly envisioned. The ingredients of the three types above can absolutely represent 100% by weight of the total weight of the compounds of the invention.

The guanidine nitrate, representing from 60 to 70% of the total weight, is in addition selected for pyrotechnic safety reasons and for its rheoplastic behavior, suitable for carrying out the compacting and optional pelletizing phases of the dry process (see hereinafter), ensuring a good densification of the starting pulverulent pyrotechnic composition while limiting the compressive load to be applied. The manufacture of the compounds via the dry process comprises up to four main steps (see hereinafter), which have in particular been described in patent application WO 2006/134311.

The potassium perchlorate is present, in the composition of the compounds of the invention, in a moderate intermediate content (from 26 to 33% by weight, advantageously from 26 to 30% by weight), very particularly with reference to the combustion temperature, the “ignitability” and the combustion rate at high pressure that are targeted.

Within the GN+KClO₄ mixtures, the combustion modifiers, selected by the inventors, develop particularly advantageous (unexpected) properties with reference to the desired three points of improvement (see above).

Said at least one combustion modifier is chosen from transition metal oxides, the precursors of such oxides and mixtures thereof. A precursor of such an oxide results in the formation of such an oxide (generates such an oxide) at the time of its decomposition at temperature during the combustion of the pyrotechnic compound. Thus, the basic copper nitrate (Cu(NO₃)₂.3Cu(OH)₂) decomposes to copper oxide (CuO) (see hereinafter).

Said at least one combustion modifier is present in an amount which is sufficient (≧2.5% by weight) to be effective (with reference to the three points of improvement above), and not excessive (≦6% by weight) so as not to harm the gas yield. Such one combustion modifier is generally present, but the presence of at least two such additives is expressly envisioned in the scope of the present invention.

Preferably, said at least one combustion modifier is chosen from zinc oxide (ZnO), iron oxide (Fe₂O₃), chromium oxide (Cr₂O₃), manganese dioxide (MnO₂), copper oxide (CuO), basic copper nitrate (Cu(NO₃)₂.3Cu(OH)₂) and mixtures thereof. The copper oxide and the basic copper nitrate, which is a precursor of said copper oxide (in the sense that BCN results in the formation of copper oxide CuO at the time of its decomposition at temperature), are particularly effective. Particularly preferably, the compounds of the invention therefore contain, as combustion modifier, copper oxide and/or basic copper nitrate. The use of these combustion modifiers makes it possible to obtain compounds of the invention which have a pressure exponent value of less than or equal to 0.1 over the pressure range 6-52 MPa.

Preferably, said at least one combustion modifier according to the invention has a specific surface area of greater than 3 m²/g, advantageously greater than 10 m²/g and very advantageously greater than 25 m²/g.

It is understood that the function of said at least one specific combustion modifier (chosen from transition metal oxides, precursors thereof and mixtures thereof) within the composition of the compounds of the invention is not only, as in the prior art (see in particular the teaching of U.S. Pat. No. 6,893,517 recalled above), to increase the combustion rate at high and medium pressure but also, surprisingly, to confer on the pyrotechnic compounds:

-   -   a stable and self-sustaining combustion at low pressure (or even         at a pressure virtually equal to atmospheric pressure),     -   a combustion rate at low pressure that is higher than that of         the prior art compositions,     -   a low, or even virtually zero, pressure exponent at low, medium         and high pressure, that is significantly lower than that of the         prior art compositions, this being with “good ignitability” of         said compounds, without generating too many solid particles at         combustion, and a combustion temperature of around 2300 K.

It may be indicated here that the compounds of the invention, the composition of which was specified above, have:

-   -   a combustion temperature of less than 2350 K,     -   a (relative, i.e. with reference to atmospheric pressure)         cut-off combustion pressure of less than or equal to 1.5 MPa,         advantageously less than 0.2 MPa and very advantageously equal         to 0.1 MPa,     -   a pressure exponent of less than or equal to 0.25,         advantageously less than or equal to 0.2 and very advantageously         less than or equal to 0.1, for a pressure between 6 and 52 MPa,     -   a combustion rate:         -   greater than 24 mm/s, advantageously greater than 36 mm/s,             at low pressure,         -   greater than 30 mm/s, advantageously greater than 35 mm/s,             at medium pressure,         -   greater than 37 mm/s, advantageously greater than 45 mm/s,             at high pressure.

The low, or even very low, pressure exponent values of the compounds of the invention must be emphasized here.

The best results indicated above (advantageous variants and very advantageous variants) were in particular obtained with copper oxide and basic copper nitrate as combustion modifier. In support of this assertion, reference may be made to the examples hereinafter.

In the context of the present invention, an original use (most particularly with reference to the above parameters) is therefore proposed for the oxides and oxide precursors in question, in the composition of the compounds of the invention (said use being original with respect to the known conventional use of a ballistic catalyst in different compositions).

In addition to the above constituents (GN+KClO₄+at least one combustion modifier of the specified type), the pyrotechnic compounds of the invention can contain, at a low content by weight (less than or equal to 6%, generally at least 0.1%), at least one additive, in particular at least one additive that facilitates the obtaining of said compounds (the forming during the obtaining thereof), such as calcium stearate or magnesium stearate, graphite and/or at least one additive for improving the aggregation of the solid products of their combustion, chosen from refractory oxides with a softening or melting point adapted to the composition, such as silica or alumina. This is advantageously silica, generally introduced in fine pulverulent form (advantageously of micrometric size, very advantageously of nanometric size) having a high specific surface area (advantageously of 100 m²/g or more), or in the form of silica fibers of small diameter (1 to 20 microns) and some tens or hundreds of microns (20 to 500 microns) in length. Surprisingly, it proved to be the case that the presence, in the pyrotechnic compounds of the invention, of silica at contents between 0.5 and 6% by weight, advantageously between 0.5 and 3.5% by weight, also has a very significant effect of drop in the cut-off combustion pressure.

It is therefore also to the credit of the inventors to have demonstrated this effect of silica in compositions of GN+KClO₄ type (see table 3 hereinafter) and therefore in the compositions of the invention (of the type GN+KClO₄+at least one combustion modifier), where said effect comes on top of those (encompassing that of the drop in the cut-off combustion pressure) of the at least one combustion modifier present.

The at least one additive intervenes with the constituent ingredients (GN, KClO₄+at least one combustion modifier of the above-mentioned type) (at the beginning of the manufacturing process) or is added, further downstream, in the process for manufacturing the compounds of the invention.

It is recalled that the compositions of the compounds of the invention do not contain an explosive ingredient (see the NF standard and the UNO recommendations specified above), this being in particular with reference to the parameters: pyrotechnic safety and combustion temperature. It is, moreover, noted that the weights of pyrotechnic compounds required for the inflation of an airbag, in particular of a side airbag, are greater than those required for the inflation of a seatbelt tensioner device according to U.S. Pat. No. 6,893,517 (said inflations not being of the same type: inflation time greater than 10-20 ms/per pulse).

The pyrotechnic compounds of the invention can be obtained according to a wet process. According to one variant, said process comprises the extrusion of a paste containing the constituents of the compound. According to another variant, said process includes a step of placing all the (or some of the) main constituents in aqueous solution, comprising solubilization of at least one of said main constituents (oxidizing agent and/or reducing agent), and then the production of a powder by spray drying, the addition to the powder produced of the constituent(s) that were not placed in solution, and then the forming of the powder in the form of objects via the usual dry processes.

The pyrotechnic compounds of the invention can also be obtained by dry process, for example by simple pelletizing of the powder obtained by mixing of their constituents.

The preferential process for obtaining the pyrotechnic compounds of the invention includes a step of dry compacting of a mixture of the constituent ingredients in powder form of said compounds (except for said at least one additive which can be added during the process). The dry compacting is generally carried out, in a manner known per se, in a roll compacter, at a compacting pressure of between 10⁸ and 6×10⁸ Pa. It can be carried out according to different variants (with a characteristic step of “simple” compacting followed by at least one additional step, with a characteristic step of compacting coupled to a forming step). Thus, the pyrotechnic compounds of the invention are capable of existing in various forms (in particular along the manufacturing process resulting in the final compounds):

-   -   at the end of dry compacting coupled with forming (by using at         least one compacting roll, the external surface of which has         cavities), plates are obtained with relief patterns, which can         be broken to directly obtain formed pyrotechnic objects;     -   at the end of dry compacting followed by granulation, granules         are obtained;     -   at the end of dry compacting followed by granulation then         pelletizing (dry compression), pellets are obtained;     -   at the end of dry compacting followed by granulation and then         mixing of the granules obtained with an extrudable binder and         the extrusion of said binder loaded with said granules, extruded         monolithic blocks (loaded with said granules) are obtained.

The pyrotechnic compounds of the invention are therefore in particular capable of existing in the form of objects of the following type:

-   -   granules,     -   pellets,     -   monolith blocks.

In a manner which is in no way limiting, it may be indicated here that:

-   -   the granules of the invention generally have a particle size (a         median diameter) of between 200 and 1400 μm (and also an         apparent density of between 0.8 and 1.2 cm³/g);     -   the pellets of the invention generally have a thickness of         between 1 and 3 mm.

When the compounds of the invention are obtained by a dry process, the constituent ingredients of the compounds of the invention advantageously have a fine particle size, of less than or equal to 20 μm. Said particle size (value of the median diameter) is generally between 3 and 20 μm. The compounds described in the present invention express all their potential if they are obtained by a dry process from powders having a median diameter of between 10 and 20 μm for KClO₄ and 5 to 15 μm for guanidine nitrate.

According to another of its subjects, the present invention relates to a pulverulent composition (mixture of powders), which is a precursor of a compound of the invention, the composition of which therefore corresponds to that of a compound of the invention (see above).

According to another of its subjects, the present invention relates to the gas generators containing at least one pyrotechnic compound of the invention. Said generators are perfectly suitable for airbags, in particular side airbags (see above).

It is now proposed to illustrate the invention in a manner that is in no way limiting.

Table 2 hereinafter gives examples of compositions of compounds of the present invention, and also the performances of said compounds compared with those of the reference prior art compound 1. The compounds were evaluated by means of thermodynamic calculations or on the basis of physical measurements carried out on granules or pellets manufactured from the compositions via the dry process of mixing of powders—compacting—granulation—and optionally pelletizing.

The reference prior art compound 1 (see table 1 above) contains guanidine nitrate and potassium perchlorate and does not contain any combustion modifier within the meaning of the invention. The compounds of examples 1 to 7 contain such a combustion modifier in their composition, in addition to the two constituents of the reference compound 1.

The amounts of the major constituents were adjusted in order to preserve an oxygen balance value close to −3%, so as to be able to directly compare the performances of the compounds of table 1.

The results of table 2 show, as expected according to the teaching of the prior art (teaching of U.S. Pat. No. 6,893,517 and that of the propellant field), that the addition of a combustion modifier within the meaning of the invention to a composition of the type of that of the reference compound 1 results in an increase in the combustion rate at medium and high pressure without any significant modification of the combustion temperature.

Surprisingly, said addition results jointly in a very large drop in the pressure exponent, which pressure exponent is very low over the entire operating pressure range (beyond 6 MPa) and in the cut-off combustion pressure and in a considerable increase in the combustion rate at low pressure.

CuO is the compound which, when added to the reference composition 1, provides the most significant improvements (see example 2). The pressure exponent is virtually zero over the whole of the operating range, the cut-off operating pressure is virtually equal to atmospheric pressure.

Insofar as a metal complex such as BCN decomposes during exothermic combustion reactions, generating, in situ, CuO with a high specific surface area (which was verified experimentally), CuO can therefore be replaced with BCN as combustion modifier, with results which are equivalent to those of CuO (see example 7).

CuO and BCN make it possible, when they are incorporated in a low amount (5% in the examples), to preserve an advantageous gas yield value (>32 g/mol) and result, in the end, in a very significant improvement in the inflation rate per unit area value (of more than 40%) compared with the GN/KClO₄ reference composition of the reference compound 1.

TABLE 2 Examples Ref. 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex 7 Ingredients Guanidine nitrate % 68.0 65.0 65.7 65.7 65.0 65.0 67.3 66.7 Potassium perchlorate % 32.0 30.0 29.3 29.3 30.0 30.0 29.7 28.3 Zinc oxide (ZnO) % — 5.0 — — — — — — Copper oxide (CuO) % — — 5.0 — — — — — Manganese oxide (MnO₂) % — — — 5.0 — — — — Chromium oxide (Cr₂O₃) % — — — 5.0 — — — Iron oxide (Fe₂O₃) % — — — — — 5.0 — Basic copper nitrate (BCN) % — — — — — — 3.0 5.0 Characteristics Combustion rate at 8 MPa (low pressure) mm/s 20.6 26.7 36.2 24.9 27.0 25.8 34.5 38.1 Combustion rate at 20 MPa (medium pressure) mm/s 26.3 31.9 38.0 31.5 31.1 31.2 36.4 39.3 Combustion rate at 50 MPa (high pressure) mm/s 34.9 38.0 45.7 39.5 37.8 41.5 44.7 47.8 Combustion temperature K 2351 2237 2303 2285 2304 2294 2312 2296 Pressure exponent determined over range — 0.26 0.16 0.07 0.19 0.17 0.25 0.09 0.10 6-52 MPa Cut-off combustion pressure (relative) MPa 1.7 1.5 0.1 1 0.5 0.25 0.1 0.1 Oxygen balance % −3.0 −2.8 −3.1 −3.1 −2.8 −2.8 −3.1 −3.1 Density g/cm³ 1.67 1.73 1.72 1.72 1.73 1.73 1.69 1.70 Gas yield at 1 bar - 1000 K mol/kg 33.2 31.6 32.1 32.1 31.6 31.6 33.1 33.0 Inflation rate per unit area (ρ. n, T, V_(c)) mol · K/cm² · s 344 391 485 397 391 390 470 505 at 20 MPa

Table 3 hereinafter shows the second surprising effect demonstrated by the inventors, namely the very significant decrease in the cut-off combustion pressure (measured on granules) when silica is introduced at a moderate content into the composition of the compounds of the invention. This same effect, obtained with another refractory metal oxide such as alumina, is not of a sufficient size to be of interest.

TABLE 3 Examples unit Ref. 1 Ex. 8 Ex. 9 Ingredients Guanidine nitrate % 68.0 65.0 65.0 Potassium perchlorate % 32.0 30.0 30.0 Alumina % —  5.0 — Silica % — —  5.0 Characteristic Cut-off combustion pressure (relative) MPa  1.7  1.5  0.5 

1. A solid pyrotechnic gas generator compound, the composition of which contains: guanidine nitrate, and potassium perchlorate, characterized in that its composition, expressed in weight percentages, contains from: 60 to 70% of guanidine nitrate, 26 to 33%, advantageously 26 to 30%, of potassium perchlorate, 2.5 to 6% of at least one combustion modifier chosen from transition metal oxides, the precursors of such oxides and mixtures thereof, 0 to 6% of at least one additive, and does not contain an explosive ingredient.
 2. The compound as claimed in claim 1, wherein its composition consists of said guanidine nitrate, potassium perchlorate, at least one combustion modifier and optionally at least one additive.
 3. The compound as claimed in claim 1, wherein its composition is at least 94% by weight, advantageously at least 98% by weight, or even 100% by weight, made up of said guanidine nitrate, potassium perchlorate and at least one combustion modifier.
 4. The compound as claimed claim 1, wherein said at least one combustion modifier is chosen from zinc oxide (ZnO), iron oxide (Fe₂O₃), chromium oxide (Cr₂O₃), manganese dioxide (MnO₂), copper oxide (CuO), basic copper nitrate (Cu(NO₃)₂.3Cu(OH)₂) and mixtures thereof.
 5. The compound as claimed in claim 1, wherein said at least one combustion modifier consists of copper oxide and/or basic copper nitrate.
 6. The compound as claimed in claim 5, which has a pressure exponent of less than or equal to 0.1, for a pressure of between 6 and 52 MPa.
 7. The compound as claimed in claim 1, wherein said at least one combustion modifier has a specific surface area of greater than 3 m²/g, advantageously greater than 10 m²/g and very advantageously greater than 25 m²/g.
 8. The compound as claimed in claim 1, wherein its composition contains silica as additive.
 9. The compound as claimed in claim 8, wherein said silica is in pulverulent form having a high specific surface area, advantageously of 100 m²/g or more, and of micrometric size, advantageously of nanometric size, or in the form of silica fibers of from 1 to 20 microns in diameter and from 20 to 500 microns in length.
 10. The compound as claimed in claim 1, wherein it is obtained by means of a dry process, which comprises a step of compacting a pulverulent mixture containing the constituent ingredients of said compound in powder form, optionally followed by a granulation step, itself optionally followed by a step of forming by pelletizing.
 11. The compound as claimed in claim 1, wherein it is in the form of granules, pellets or monolithic blocks.
 12. A pulverulent composition, which is a precursor of a compound as claimed in claim 1, the composition of which corresponds to that of a compound as claimed in claim
 1. 13. A gas generator, suitable for an airbag, which contains at least one compound as claimed in claim
 1. 